Electronic systems are increasingly used in harsh environments under elevated thermal conditions. This includes applications and products subject to high absolute temperatures as well as very large temperature swings, such as for example deep well drilling (e.g., geothermal, oil or natural gas), military and aerospace, and automotive applications and products.
The secure interconnection of electronic components and materials allows for proper functionality and durability of electronic systems. However, interconnect failure is a major obstacle in the design of reliable electronic systems (McCluskey, P. “Reliability of Power Electronics Under Thermal Loading,”7th International Conference on Integrated Power Electronics Systems, Mar. 6-8, 2012). Electric interconnects must provide a multitude of functions. For example, in power electronic applications, these include: mechanically affixing an integrated circuit or chip to a substrate; providing an interface with low thermal resistance for efficient heat transfer from chip to heatsink; and providing an interface with low electrical resistance with high current-carrying capability.
A typical power module structure is shown in FIG. 1, which includes a die attached to a Direct Bond Copper (DBC) substrate via a conventional interconnect or attach material. The DBC is also attached to a copper base, which acts as a heatsink. Significant thermo-mechanical stresses occur within attach layers during operation due to significant differences in the coefficient of thermal expansion (CTE) between the die and the DBC, as well as between the DBC substrate and the heat sink. Harsh environments coupled with growing power densities lead to higher joint temperatures that soften conventional attach materials, making them susceptible to fatigue and reducing their system time-to-failure or in extreme cases, leading to melting of the attach material. Furthermore, the thermo-mechanical stresses increase with increasing temperatures. Conventional attach materials utilized in power electronics were developed for application temperatures between about 100° C. and 150° C. Such conventional attach materials include solders with melting temperatures (Tm) in the range of between about 200° C. to 300° C. As such, they are not suitable for applications at elevated temperature conditions (e.g., such as 200° C. or higher).
In automotive applications, the integration of microelectronic systems in the engine compartment, referred to as the under-the-hood environment, increases thermal load on such systems due to heat dissipated by the engine during operation. With the commercial success of electric vehicles (EV) and hybrid electric vehicles (HEV), power electronic systems for energy storage and conversion are used ubiquitously in automotive applications. With temperatures of the liquid coolant of 75° C. and more, to ensure reliable operation of silicon (Si) power devices at temperatures below 150° C., power densities have to be limited, or considerable effort concerning the thermal management systems is required, e.g. such as by providing a secondary liquid cooling loop. However, additional cost as well as increased weight and volume requirements are associated with these efforts.
Another industry with continually increasing requirements for electronic systems is the aerospace industry, wherein systems are routinely exposed to severe conditions. Planned space explorations require that electronic systems are capable of withstanding extreme pressures, stresses and temperatures. For example, the exploration of Venus will require that systems are capable of withstanding an atmospheric pressure more than 90 times greater than that on earth, temperatures of 467° C. or higher, and highly corrosive sulfuric acid. Thus, applications in such severe environmental conditions, coupled with increasing power densities and miniaturization necessitate the need for electronic systems capable of functioning at higher temperatures.
Traditional Si devices are limited to relatively low voltage operation, and are not suitable for use at temperatures above 175° C. due to their material properties as a low band-gap semiconductor. Therefore, devices based on the wide band-gap (WBG) semiconductor materials (e.g., such as silicon carbide (SiC) and gallium nitride (GaN)) have been introduced. Compared to traditional Si devices, SiC and GaN devices may be operated at higher temperatures, exhibit higher heat dissipation, possess higher breakdown voltages, and enable higher switching frequencies with reduced switching losses. Multiple systems operating in temperature ranges up to 200° C. and above have been presented. For example, the operation of a DC-DC converter based on SiC junction gate field-effect transistors (JFETs) and Schottky barrier diodes (SBDs) was demonstrated at temperatures up to 450° C. (Funaki et al. (2007) “Power Conversion with SiC Devices at Extremely High Ambient Temperatures,” IEEE Transactions on Power Electronics, vol. 22, no. 4).
Thus, SiC has become the semiconductor material of choice for manufacturing many power electronic devices. Devices made with SiC can block more voltage in the reverse bias direction because of its inherently larger breakdown strength. In addition, SiC permits devices to have a faster switching speed, thereby allowing operation at a higher frequency because of its higher saturated electron drift velocity. SiC also provides better heat transfer because of its higher bulk thermal conductivity. Further, SiC permits higher temperature operation, because of its larger bandgap. This ability to operate at higher temperatures permits devices to be operated at higher power with higher dissipation without requiring more cooling capacity. It is, however, this operation at higher temperature that has created a major packaging challenge.
Packaging and interconnect technologies applied in conventional microelectronic systems were developed for traditional Si-based devices, which are limited to relatively low temperature applications (e.g., temperatures below 200° C.) (see Raynaud et al. (2010) “Comparison of High Voltage and High Temperature Performances of Wide Bandgap Seminconductors for Vertical Power Devices,” Diamond and Related Materials 19(1):1-6; Scofield et al. (2010) “Performance and Reliability Characteristics of 1200V, 100A, 200° C. Half-Bridge SiC MOSFET-JBS Diode Power Modules,” International Conference on High Temperature Electronics, May 2010, Albuquerque, N. Mex.; see also Xu et al. “Investigation of Si IGBT operation at 200° C. for traction application,” Energy Conversion Congress and Exposition (ECCE), 2011 IEEE, Sep. 17-22, 2011, Phoenix, Ariz.; Xu et al. “Characterization of a High Temperature Multichip SiC JFET-Based Module,” Energy Conversion Congress and Exposition (ECCE), 2011 IEEE, Sep. 17-22, 2011, Phoenix, Ariz.; Ryu et al. (2012) “Ultra High Voltage (>12 kV), High Performance 4H-SiC IGBTs,” Proc. Int. Symp. Power Semiconductor Devices and ICs, ISPSD'2012, pp. 257-260).
For example, eutectic Sn37Pb solder alloys have traditionally been utilized as interconnect or attach material in electronic systems. However, they have a melting temperature (Tm) of 183° C., which is far too low for high temperature applications. Additionally, eutectic Sn37Pb solders show relatively high creep rates at elevated temperatures and are subject to many regulatory restrictions. As such, they are not suitable for many applications.
Eutectic Sn3.5Ag and SAC305 (Sn—Ag—Cu) alloys also possess comparatively low melting temperatures of 221° C. and 217° C., respectively. They are frequently used as substitute materials for Sn37Pb solder (which is being phased out by regulatory restrictions) due to their similar Tm and wetting behavior. However, similar to other lead (Pb)-based solders, they are highly ductile and soften close to their Tm, limiting their fatigue life. They are therefore utilized for low temperature applications.
Bi—Ag alloys, while less conventional, are another reflow solder interconnect, which possess liquidus temperatures of 262° C. However, they exhibit brittle behavior with limited ductility and elongation. Such problems can be slightly mitigated by increased ratios of silver (Ag), but at substantially higher cost. Moreover, they possess limited wetting capabilities and low thermal conductivities. As such, Bi—Ag solders have not proven reliable or efficient for many applications.
Solders with Pb as the main constituent (e.g., such as Pb5.0Sn2.5Ag) have melting temperatures of 296° C. Their use is restricted by regulations for most applications, with a few exceptions such as military applications and as die attach. As such, their availability has been substantially reduced due to the shrinking market and the possibility they will be entirely restricted by future regulations (e.g., such as the Restriction of Hazardous Substances Directive). Moreover, they require relatively high processing temperatures, and exhibit high creep rates at elevated temperatures.
A few alloys with gold (Au) as a main constituent are available, including Au20Sn, Au12Ge, and Au3.2Si, with melting temperatures of 280° C., 361° C., and 363° C., respectively. They form relatively strong joints with good fatigue life, but must be processed at high temperatures. Moreover, relatively high costs are associated with these alloys, which inhibit their use in many applications.
Other solder alloys provide zinc (Zn) as the main constituent, including Zn6Al and Zn5.8Ge, with melting temperatures of 381° C. and 390° C., respectively. However, such alloys also require relatively high process temperatures, with complicated and expensive processing techniques.
Suitable interconnect materials and processing techniques therefore remain a major challenge in the design of reliable high-temperature packages for power electronic devices and systems. Wide temperature swings and high temperatures substantially increase thermo-mechanical stresses imparted on a device. At elevated temperatures, the solder strength decreases while deformation or creep accelerates, resulting in increased deformation during each load cycle and a reduction in fatigue life. Established solder technologies have failed to overcome such problems and to provide for reliable high temperature operation.
Thus, there is a need for alternative interconnect materials, as well as alternative application and processing methods, which overcome some or all of the above-noted problems.